Mercury is a fascinating material.
Even I enjoy playing with it.
But like most heavy metals, it is poisonous.
It's quite surprising to people, if you have this liquid running around in a dish or something, how can it possibly poison you if you're not touching it?
The answer is that there is vapour of mercury coming off, which you can breathe.
And, remarkably, it's quite easy to show you how this vapour comes off.
Before we show you the experiment,
I have to explain a little bit about mercury lamps.
UV light can be generated by filling a tube with a small amount of mercury
and then passing through an electric current.
The electric current heats the mercury to a very high temperature
and the mercury atoms give out ultraviolet light.
This is light we can't see.
And the ultraviolet light is at a very precise wavelength,
around 254 nanometers.
So now, Neil has one of these lamps
quite an old one, but it still works.
And let's see what happens when he shines it
across the surface of a dish of mercury.
To make the effect a bit easier to film,
he's warmed the mercury a bit, but it's not very warm.
Probably less than 100 degrees Centigrade
like a cup of tea, or something like that.
We, as humans, cannot see UV light.
Our eyes don't detect it.
In order to see the output of the lamp
you have to shine it on a sheet of paper
which contains a dye to make it look white.
You know, paper is usually a bit brown.
And that dye absorbs the UV light
and gives out a sort of purplish color of light.
So that's why the piece of paper that Neil is holding
is glowing purple.
In between the lamp and the paper
there is vapour coming off the mercury.
Not very much, but the mercury atoms in the vapour
can absorb precisely the same wavelength of light
as the lamp is giving out.
So it acts as a very precise and powerful filter
for the light. So the paper doesn't glow as brightly.
And you can see these clouds of swirling vapour.
And those clouds are the mercury vapour
which, if you had your nose close to it,
you would be breathing in.
And that's why liquid mercury is dangerous.
Because, all the time, vapour is coming off.
And mercury is one of the elements which can accumulate
in biological systems like your body.
So even if the mercury is coming off slowly
if you're exposed to it a long time,
you can absorb quite a lot.
And there's stories of old lab buildings, where
they used to use mercury, where, when they're doing
the refurbishment, they lift the floorboards
and underneath there're pools of mercury
which have fallen through when people have spilled it
over the years.
In some of these places,
there is quite a high level of mercury vapour
that is coming up through the floor.
Though, usually, it's so dirty under the floor
that there's this layer of dust which prevents most of it coming off.
But, the take-home message
with this is that if you're using mercury,
and there are all sorts of experiments where it's useful
to use mercury, you must use it in a fume hood
that's well ventilated, so you don't breathe the vapour.
...from the vapour pressure in the air
and he dropped in the mercury.
What's interesting is that, as it goes into the liquid nitrogen,
the nitrogen boils, and so the mercury falls
almost encased in this sort of bubble of nitrogen gas.